Process and composition for forming clear protective coatings on cadmium metal surfaces



United States Patent O 3,298,876 PROCESS AND COMPOSITION FOR FORMING CLEAR PROTECTIVE COATINGS ON CAD- MIUM METAL SURFACES Merton M. Beckwith, Timberlake, Darryl A. Richard, North Ridgeville, and Joseph A. Zehnder, Rocky River, Ohio, assignors to R. 0. Hull 8: Company, Inc., Cleveland, Ohio, a corporation ofOhio No Drawing. Filed Sept. 9, 1963, Ser. No. 307,314 4 Claims. (Cl. 148-621) This invention relates to the treatment of surfaces of cadmium metal to form clear protective coatings thereon. More particularly, the invention concerns (1) new processes for the conversion coating of cadmium, (2) dip solutions for the treatment of cadmium metal surfaces to form clear protective coatings thereon and (3) powdered mixtures or other concentrates which may be admixed with water to form new cadmium metal passivating dips or brightening dips.

FIELD OF INVENTION Cadmium metal surfaces formed by electroplating or other coating procedures generally do not have an appearance or resistance against discoloration or corrosion necessary to provide articles which may be satisfactorily handled andsold in commerce. For example, electroplated cadmium surfaces or other freshly formed cadmium metal surfaces are readily stained, particularly by atmospheres containing small amounts of sulfur containing materials. Such surfaces are also stained by contacts with 'the hands of persons handling the articles, a form of staining which is referred to in the trade as fingerprinting. Many procedures and compositions are known that may be used for the treatment of cadmium metal surfaces to form on them protective or decorative coatings. In some cases, the known coating procedures are primarily used to create colored or decorative coatings, protective or 3,298,875 Patented Jan. 17, 1967 the relatively light coatings are intended to provide a finish which is clear or transparent and is of minimum thickness or quantity so as to require only the smallest amount of treating material for its production. Examples of this light dip form of protective coating operation are discussed in US. Patents 2,559,878 and 3,072,516.

The light dip variety of protective coating, in the past, has involved several problems. For example, such cadmium coatings are inclined to fingerprint unsatisfaccorrosion resistance aspects of the coatings being secondary. Black colored coatings such as provided by the processes described in US. Patent 2,524,577 are examples of-decorative coatings for cadmium. Other known procedures have as a primary purpose the formation of relatively heavy coatings designed to give a maximum of corrosion protection with the aesthetic appearance of the article being secondary. The use of relatively concentrated aqueous solutions of alkali metal ch-romates or dichromates is probably the most notable method of forming this type of heavy coating. Descriptions of such chromate coating processes and compositions may be found, for example, in US. Patents 2,186,579 and 2,904,414.

There is yet another general type of coating operations employed for the treatment of cadmium metal surfaces which will be referred to herein as a light dip coat. This is designed to form only a relatively light coating on the cadmium metal surface which will provide an attractive appearance that will remain in satisfactory form during normal handling and contact with relatively noncorrosive atomspheres which would usually be encountered in the storage, shipment and sale of the cadmium metal coated articles. Such light dip coatings are not designed to give the high degree of corrosion protection that can be obtained with the heavy coatings afforded by the concentrated chromate solution treatments. Thus,

torily in humid weather. Also, in atmospheres containing mild concentrations of sulfide fumes, there is a tendency for cadmium metal articles coated with such coatings to acquire an overall staining. Accordingly, there is a need for improved conversion coatings for cadmium which will eliminate the staining, fingerprinting and other problems which have been associated with prior known operations of performing light dip coatings on cadmium metal surfaces. Any improved coating procedure of this type must not create other problems, the inexpensive to perform and not require special handling or processing equipment. OBJECTS A principal object of this invention is the provision of new conversion coating processes for the treatment of cadmium metal surfaces. Further objects include:

(1) The provision of new aqueous solutions which may be used for the dip treatment of cadmium metal surfaces to produce thin, clear protective coatings thereon.

(2) The provision of dry powder compositions that may be mixed with water, with or without the addition of small amounts of mineral acid to adjust pH, to form dip solutions into which articles having cadmium metal surfaces may be immersed for relatively short periods of time to form clear protective coatings on the cadmium metal.

(3) The provision of new processes for treating cadmium metal surfaces to produce thereon clear protective coatings to protect the metal suface against fingerprinting or other staining in the course of normal handling, storage and the like without, at the same time, removing any of the cadmium metal coating that had been applied to the article or otherwise adversely effecting it.

(4) The provision of new forms of aqueous dip solutions of relatively low concentration, e.g., 1 to 10 pounds of active ingredient per hundred gallons of treating solution, which may be used for the passivating or brightening of cadmium metal articles with a minimum of expense as to treating materials and at relatively high rates of production of treated surface.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

GENERAL DESCRIPTION These objects are accomplished according to the present invention by the use of aqueous cadmium conversion coating solutions which contain as essential ingredients (a) trivalent chromium ions, (b) hexavalent chromium .0 ions, and (c) fluorine ions with the weight ratio of trivalent chromium ions to hexavalent chromium ions being about to 1 and more broadly, is between about 6:1 and 4:1. The concentration of fluorine ions in said solutions on a weight basis is higher than the concentration of either of the trivalent or hexavalent chromium ions and is preferably about 4:1, on a weight basis, relative to the trivalent chromium ions and this ratio may advantageously be between about 3:1 and 5:1.

The formation of aqueous dip solutions having such characteristics may be accomplished by the use of a variety of water-soluble chromium salts, water-soluble chromates and water-soluble fluorides. Advantageously, the light dip coating of cadmium in accordance with the invention can be carried out by mixing a dry powder composition of the materials listed below with suflicient water to provide an aqueous bath having an active ingredient concentration of about 0.1 pounds per 100 gallons of solution and particularly about 1-5 pounds per 100 gallons:

Ingredient: Parts by Weight M2Cl'2(SO4)424H2O HF 25-40 M CrO 0.1-1 MR 0.1-1

In the above table, M represents a material from the group consisting of alkali metals and ammonia and R represents the cation of an organic acid having a dissociation constant pK between about 3 and 5.

The aqueous dip solutions may be used at various pI-ls. Advantageously, the pH of the solution should be about 1.5 and 4.0. In the region of pH 1.5 to 2.0, the dip solutions give passivization as well as brightening of the cadmium metal surface whereas in the higher range, particularly a pH about 3.0 to 3.5, there is a passivization of the metal surface without any brightening effect. Normally, the addition of the mixture of metal salts will produce a solution having a relatively acid pH, i.e., about 4.0 or less, but, if necessary, the pH of the solution may be adjusted downwardly by addition of mineral acid, particularly nitric acid. In use, the dip baths tend to increase in pH and the advantageous pH range for the exact quality of coating desired can be maintained by periodic additions of nitric or other strong acid.

Conversion coating solutions as described are relatively weak as compared with alkali metal chromate compositions used to provide the heavier and more corrosion resistant coatings typical of the heavy concentration solutions, e.g., solutions containing hexavalent chromium compounds in a concentration about 25 to 100 parts per 100 gallons of treating solution. If such chromate solutions are diluted to the range of about 1 to 10 pounds of active ingredient per 100 gallons of solution, immersion of cadmium metals in such solutions only results in staining and such baths are not suitable for use as brightening or passivization dips. Weak nitric acid has, in the past, been a common light dip solution, but cadmium metal surfaces treated in such solutions are susceptible to very easy staining in hot or humid atmospheres.

The new dip coating solutions may be employed advantageously using treating conditions comparable to those generally employed in the dip coating of cadmium. Advantageously, the dip solutions are maintained at a temperature about 50 to 150 F. while the cadmium metal surface to be coated is immersed therein. The dip coatings are relatively mild and do not remove any appreciable amount of cadmium metal from the treating surface during the time necessary to create the desired protective coating on the surface. Consequently, substantially wide ladder latitude in the time of immersion is possible using these new solutions. Advantageously, the time of immersion is between about 1 to 300 seconds within the temperature range 50 to 150 F.

A more complete understanding of the procedures and compositions of this invention may be had by reference to the following examples of actual operations carried out in accordance with the invention. In these examples, all parts or percentages are by weight unless otherwise specified.

Example I A dry powder composition to serve as a coating bath concentrate was prepared by mixing together the following ingredients which had been milled to a particle size small enough to pass a standard 100 mesh per inch sieve:

Parts Potassium chrome alum K Cr (SO 24H O Sodium hydrogen fluoride NaHF 40 Sodium chromate Na CrO 0.5

Sodium acetate NaOCOCH Q. 0.25

Two pounds of the resulting powder composition were dissolved in 100 gallons of water and 0.5% by volume of nitric acid added giving a clear green colored solution having a pH of 3.5. With the solution at a temperature of F., cadmium-plated steel hardware articles held in polyethylene baskets were dipped into the treating solution and held there for about 15 seconds. The plastic, baskets and the treated cadmium-plated articles were then removed and transferred to a hot water rinse bath, removed from the rinsing bath and then allowed to dry in air. The resulting articles exhibited a subdued lustrous surface substantially identical in appearance to a freshly formed plated coating of cadmium metal.

The effectiveness of the coating to resist straining and discoloring was tested by applying one drop of a 1% lead acetate solution to the protective-coated surface and then measuring the length of time required before the spot covered by the drop of lead acetate solution became preceptably black in color. Using this test on a multitude of the hardware items treated as described, it was found that 6 to 8 seconds were required for the threshold appearance of the black coloring.

In another case, cadmium-plated hardware items of the foregoing type were immersed for 15 seconds at 70 F. in a solution formed by dissolving 6 pounds of potassium chrome alum in water and adding 0.5% by volume of nitric acid. The so treated cadmium-plated articles possessed a lustrous transparent surface after drying, but when subjected to the 1% lead acetate drop test, the drop spot turned black in all cases within one second after application of the drop.

Example I] Using a dry powder mixture as described in Example I, 2 pounds of the composition 'were dissolved in gallons of water and 1.5% by volume of nitric acid was added giving a solution with a pH of 1.6. Cadmium electroplated steel hardware items supported polyethylene baskets were lowered into the resulting dip solution at a temperature of 70 F. After being immersed for 15 seconds, the cadmium-plated articles were removed, transferred to a hot water rinse bath, rinsed for about one minute, then removed and allowed to dry in air. The resulting treated articles had a bright silvery appearing surface which Example 111 A conversion coating solution was prepared by dissolving the following ingredients in 100 gallons of water containing 1% by'volume of nitric acid:

Chromic sulfate Cr (SO l8H O pound 1 Sodium hydrogetn fluoride pound 0.8 Sodium chromate ounces 1.5

Cadmium electroplated hardware items were treated in this solution at 70 F. for 30 seconds, after which they were rinsed in water and allowed to air dry. The resulting treated articles had a surface appearance similar to the treated articles obtained in Example II. The average time for appearance of a black spot using the lead acetate test as described in Example I on these articles was -6 seconds.

DISCUSSION OF DETAILS Trivalent chromium ion providing materials are an essential ingredient to the new dip solutions and, in turn, the dry powdered mixtures or other concentrates used in the formation of such dip solutions. Potassium chrome alum is the preferred trivalent chromium providing material, particularly because of its compatibility with other components of the new compositions in the powder mixture and because of the fine lustrous or bright quality of the resulting protective coatings. In place thereof, in part or in whole, other alkali metal chrome alums or ammonium chrome alum may be used. Also, chromium sulfate or chromium fluoride may be employed.

Broadly, dry powder mixtures of ingredients to give a composition as follows may be used as dry concentrates for making dip bath in accordance with the invention:

Parts Alkali metal chrome alum 50-70 Alkali metal hydrogen fluoride 35-45 Alkali metal chromate 0.1-1

The fluoride ion of the dip solutions may be provided by hydrofluoric acid if the dip solutions are made up by dissolving individual solution ingredients in water. However, where a dry powder mixture is to be employed as the concentrate, a water-soluble fluoride salt is used, such as sodium fluoride or an acid salt such as sodium hydrogen fluoride. Somewhat similar results can be obtained by the use of sodium fluoride in combination with an acid salt such as sodium hydrogen sulfate.

The minor proportion of hexavalent chromium is advantageously furnished by the use of sodium chromate. However, other alkali metal or ammonium chromates may be used or a comparable amount of chromium trioxide or equivalent hexavalent chromium water-soluble material may be employed.

The sodium acetate is an optional ingredient which increases rinsibility of the article and acts as a buffer. Instead of sodium acetate, other water-soluble alkali metal salts or ammonium salts of organic acids having a dis sociation constant pK of between 3 and 5 may be used, e.g., sodium citrate, sodium benzoate, sodium propionate, sodium acrylate and comparable potassium or other alkali metal salts. 1

If desired, minor amounts of other ingredients may be included in the treating baths. This includes wetting agents and other surfactants, such as the wetting'agents disclosed in US. Patent 2,610,133, coating color modifying materials such as water-soluble silver and lead salts, and other modifying agents known to be useful in the art of dip solution coating of cadmium metal surfaces. Such additives will constitute a minor amount of the dip solution, i.e., 1 to 10% by weight of the total solids content of the solution and usually about 1 to 10% by weight of the trivalent chromium material, e.g., potassium chrome alum.

The dip solutions and procedures as described above may be employed to treat all types of articles which have a surface of cadmium metal thereon. This includes articles made entirely of cadmium or cadmium base alloy as well as articles made of other metal, plastic or the like having only an exterior surface of such cadmium metal.

The new procedures and compositions are particularly useful in treating ferrous-metal articles or other metal articles which have been electroplated with cadmium, but they may be used to form clear protective coatings on any other articles to which a coating or layer of cadmium metal has been applied to an exterior surface. The solutions and procedures may be employed with standard dip coating equipment using the same general operations with which the equipment operators are familiar so that introduction of the new solutions into a manufacturing operation usually requires no special training of employees. The effectiveness of the new coating operations and the relatively small amount of materials required makes these treating methods economically very attractive for the production of stain resistant clear coated cadmiumplated hardware items and other articles.

Having described the invention in detail, the subject matter to be covered by Letters Patent is defined in the following claims:

We claim:

1. A dry powder composition for admixture with water to form a solution for treating cadmium surfaces to produce thereon clear protective coatings consisting essentially of the following ingredients in the proportions by weight specified:

. Parts Potassium chrome alum 60 Sodium hydrogen fluoride 40 Sodium chromate 0.5 Sodium acetate 0.25

2. A dry powder composition for admixture with water to form a solution for treating cadmium surfaces to produce thereon clear protective coatings consisting essentially of the following ingredients present in the proportions by weight specified:

Parts Alkali metal chrome alum 50-70 Alkali metal hydrogen fluoride 25-40 Alkali metal chromate 0.1-1 Alkali metal acetate 0.1-1

3. A solution forming concentrate for admixture with water to form an aqueous treating bath in which to immerse articles having cadmium metal surfaces to produce clear protective coatings on said surfaces consisting essentially of the following ingredients in the proportions by weight specified:

Parts 'M Cr (SO 24H O 50-70 MHF 25-40 M2CI()4E MR 0.1-1

wherein M is a material selected from the group consisting of alkali metals and ammonium and R is the cation of an organic acid having a dissociation constant pK between about 3 and 4.

4. A process for treating cadmium metal surfaces to produce thereon clear protective coatings which comprises:

(A) providing an aqueous solution having a pH between about 1.5 and 4.0 containing as the essential ingredients the following materials in proportions by weight specified per 100 gallons of solution:

(B) maintaining the temperature of the immersion solution at between about 50150 F (C) immersing the cadmium metal surface to be treated in the solution for between about 1 to 300 seconds References Cited by the Examiner UNITED STATES PATENTS 2/1940 Allen 148-621 5/1950 Bruno et al. 148-621 X 7/1951 Johnson 1486.21 6/1957 Chester et a1. 148-621 FOREIGN PATENTS 3/ 1947 Great Britain.

' ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner. 

1. A DRY POWDER COMPOSITION FOR ADMIXTURE WITH WATER TO FORM A SOLUTION FOR TREATING CADMIUM SURFACES TO PRODUCE THEREON CLEAR PROTECTIVE COATINGS CONSISTING ESSENTIALLY OF THE FOLLOWING INGREDIENTS IN THE PROPORTIONS BY WEIGHT SPECIFIED: 